After studying about effective potentials, I understand that circular orbits occur when the total energy of the orbiting body is a minimum, elliptical orbits when the total energy is between this value and zero, and so on so forth. My question is, what happens when the total energy is less than the energy of a circular orbit? For example, a spaceship in circular motion applying its brakes. My intuition tells me it will spiral towards the body it is gravitating about, and I understand that the eccentricity of the orbit will become imaginary, but I don't know if this intuition is correct, and I haven't found any literature online to confirm/debunk my understanding.
Edit: I can understand that the effective potential changes when the speed of the orbiting body changes, since the angular momentum changes. My confusion lies with the total energy. So, I understand that total energy comprises the gravitational potential energy and the kinetic energy. In a circular orbit, this total energy is a minimum of $-GMm/2r_0$. When the body applies brakes and slows down, at that instant, the gravitational potential energy remains constant and the kinetic energy decreases. What happens afterwards is what my brain cannot comprehend.